This invention relates to an infinitely variable transmission which is connected at its input end to a hydrodynamic transmission unit such as fluid coupling or a torque convertor.
Infinitely variable transmissions of the kind contemplated by the present invention include an endless belt or chain running over a primary and a secondary V-shaped pulley, each pulley being constructed of two conical discs, one conical disc of the primary pulley being driven by a power source and one conical disc of the secondary pulley driving the transmission output shaft. The other conical disc of each pulley is axially adjustable so that the running diameters of the pulleys can be changed in order to change the ratio of the transmission. Adjustment of the two axial positions of the axially movable discs is effected simultaneously in a manner such that movement of the disc of the second pulley compensates for movement of the primary pulley disc. Control of the axial movement of the adjustable pulley discs is obtained by controlling hydraulic pressure within a hydraulic operating cylinder associated with each pulley, the adjustable disc being integral with or connected to a piston movable within the cylinder. The control device for the cylinders includes a hydraulic spool valve which controls the supply and exhaust of pressure fluid to the cylinders. The valve includes an axially movable spool which is biased in one direction by a fluid pressure which, for example through a Pitot tube, is built up in proportion to the speed of the transmission input shaft, i.e. in proportion to engine speed. The spool is biased in the opposite direction by a compression spring which in turn is biased in proportion to, for example engine intake manifold pressure or the position of the engine throttle.
More specifically the present invention relates to a transmission of the above-summarized kind wherein the valve spool can be displaced from an end position where the valve spool rests against a stop and where ports are opened for the discharge of hydraulic fluid from the primary operating cylinder and/or the supply of liquid to the secondary operating cylinder, to a center position where ports are substantially closed, so that virtually no supply of hydraulic fluid to and discharge of hydraulic fluid from at least one of the operating cylinders can take place.
Such a transmission, which is known from, for instance, U.S. Pat. No. 4,152,947, can be used for driving a motor vehicle. To this end, the transmission may also be provided with a hydrodynamic transmission unit, such as a fluid coupling or a torque converter, connected between the input to the primary pulley and the power source. Such a unit permits the vehicle to drive away smoothly from standstill and also, in the case of a torque converter, to increase the torque produced by the engine when driving away.
A drawback of such a hydrodynamic transmission unit is, however, that it contributes to loss of power in transmission in that the input speed of the hydrodynamic transmission unit generally exceeds the output speed. To reduce this loss of power, the transmission may be provided with lock-up means for bridging the hydrodynamic transmission unit, in which the input and the output shaft of this unit are linked directly to each other by mechanical means. Such a lock-up can be actuated when the hydrodynamic transmission unit is not used for driving away or increasing the torque.
In Netherlands patent application No. 7807579 it has been proposed to make the lock-up operable in dependence on the fluid pressure in the primary operating cylinder. This design results in the lock-up being actuated as soon as the variable V-belt transmissin does not have the maximum transmission ratio, i.e. as soon as the transmission is not in the lowest gear. In consequence, when the vehicle is driving away from standstill the hydrodynamic unit is by-passed (bridged) as soon as the transmission, which initially is in the lowest gear, starts to shift up.
Such a simply operating lock-up does not, however, satisfy for every type of hydrodynamic unit, especially when large differences may occur between the input and the output speed. In that event, a so-called overshoot arises after the vehicle is driving away, that is, for a short moment the engine speed rises above the predeterminated value of the engine speed during shifting-up of the transmission.